The invention concerns improvements in and relating to the antistatic properties of coating compositions of vinylidene chloride copolymers used as coatings on organic polymeric films.
It is well known in the art to apply coating compositions of the above-specified type to various base films so as to achieve combinations of properties not possessed by either the base film or coating alone. Thin films, however, are prone to develop static charges which cause the films to stick and jam in machinery used for packaging and laminating operations. In order to meet such problems, it is known to add to coating compositions small amounts of substances which confer antistatic properties, or to aftersize coated films with similar materials.
For example, U.S. Pat. No. 3,677,811 discloses the incorporation of an antistatic composition comprising a mixture of a compound such as bis(2-hydroxyethyl) myristyl amine or bis(2-hydroxyethyl) tallow amine and one or more glycerides into a coating of a heat sealable polymer that is applied to a base film.
Nevertheless, static problems remain in many instances. To cite but a single example, solvent coating of polyester films with vinylidene chloride copolymer (sometimes referred to as saran) coating compositions results in films having a very high static propensity. Although antistats have been added to the coating composition, they are either not sufficiently effective to eliminate static build-up, or they adversely effect other properties such as the heat seal strength, especially at high humidities. Saran-coated film can be aftersized to lower the static propensity, but this has the disadvantages that the expense of an extra coating operation is required, and if too much aftersize is used, the heat seal strength is lowered to an unacceptable level. Also, in the case of aftersizing a coated polyester film, the film tensions required in the drying section of the coater result in increased film shrinkage upon subsequent reheating.